[RISCV] matchSplatAsGather should be disable for i1 vectors

[topperc]

This DAG combine will create an unsupported VRGATHER if it runs for a mask vector.

@frasercrmck

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[topperc]

Candidate patch 56007

[lukel97]

This was fixed in e406252